1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More particularly, the present invention relates to a method for fabricating a liquid crystal display (LCD) device to improve picture quality by preventing defective rubbing.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have attracted great attention as a substitute for a Cathode Ray Tube (CRT), because of the advantageous characteristics of the LCD such as thin profile, light weight, and low power consumption. The LCD device is driven by changing optical anisotropy in a method of applying an electric field to liquid crystal having fluidity and optical characteristics.
The LCD device has various modes based upon the properties and type of liquid crystal used within the device. Specifically, the LCD device may be may categorized as a Twisted Nematic (TN) mode that controls the liquid crystal director by applying a voltage after arrangement of the liquid crystal director twisted at 90°, a multi-domain mode that obtains a wide viewing angle by dividing one pixel into several domains, an Optically Compensated Birefringence (OCB) mode that compensates a phase change of light according to a progressing direction of light by forming a compensation film on an outer surface of a substrate, an In-Plane Switching (IPS) mode that forms an electric field parallel to two substrates by forming two electrodes on one substrate, and a Vertical Alignment (VA) mode that arranges a longitudinal (major) axis of liquid crystal molecule vertical to a plane of an orientation layer by using negative type liquid crystal and vertical orientation layer.
Generally, a LCD device includes an upper substrate of a color filter array, a lower substrate of a thin film transistor array, and a liquid crystal layer. The upper and lower substrates face each other, and a liquid crystal layer having dielectric anisotropy is formed between the two substrates. To use the LCD device as an optical device, it is necessary to align liquid crystal molecules of the liquid crystal layer at a predetermined direction. Accordingly, the orientation layer, organic polymer layer, is formed on the substrate, wherein the orientation layer has anisotropy by rubbing. The LCD device may be classified as a transmission type LCD device using a backlight as a light source, a reflective type LCD device using ambient light as a light source without forming the backlight, and a transflective type LCD device which overcomes the disadvantageous characteristics of the transmitting and reflective type LCD devices. The transmission type LCD device has the disadvantage of high power consumption due to the backlight, and the reflective type LCD device has the problem that it cannot be used in the dark surroundings.
The transflective type LCD device has both transmission and reflective parts in a unit pixel, whereby the transflective type LCD device serves as the transmitting or reflective type LCD device as needed. Accordingly, a pixel electrode may be formed as a transmitting electrode or a reflective electrode according to the kind of the LCD device. For example, the transmitting electrode may be formed in the transmission type LCD device and the transmitting part of the transflective type LCD device. Also, the reflective electrode may be formed in the reflective type LCD device and the reflective part of the transflective type LCD device. The transmitting electrode of the transmission type LCD device and the transflective type LCD device transmit light emitted from the backlight through a lower substrate to the liquid crystal layer, to obtain high luminance. Also, the reflective electrode of the reflective type LCD device and the transflective type LCD device reflects ambient light incident through an upper substrate, to obtain high luminance.
A related art IPS mode LCD device will now be described as follows. FIG. 1 is a plane view illustrating an IPS (In-Plane Switching) mode LCD device according to the related art. FIG. 2 illustrates voltage distributions of an EPS mode LCD device according to the related art. FIG. 3A and FIG. 3B are plane views illustrating an IPS mode LCD device when a voltage is turned on/off.
In the related art IPS mode LCD device, as shown in FIG. 1, gate and data lines 12 and 15 crossing each other are formed on a substrate to define a pixel region, and a common line 24a is formed within the pixel region substantially in parallel to the gate line 12. Also, a thin film transistor TFT is formed at a crossing portion of the gate and data lines 12 and 15, and a common electrode 24 diverged from the common line 24a is formed substantially in parallel to the data line 15 within the pixel region. A pixel electrode 17 is connected with a drain electrode of the thin film transistor TFT, and formed substantially in parallel between the common electrodes 24. Also, a storage electrode 25 extending from the pixel electrode 17 is formed on the gate line 12.
In the aforementioned IPS mode LCD device, if 5V is applied to the common electrode 24, and 0V is applied to the pixel electrode 17, as shown in FIG. 2, an equipotential surface is formed substantially parallel to the electrode at a portion corresponding to the electrode, and the equipotential surface is formed vertical to the electrode at a portion between the two electrodes. In this state, because a direction of an electric field is perpendicular to the equipotential surface, a substantially parallel electric field is generated between the common electrode 24 and the pixel electrode 17, a vertical electric field generates on the electrode, and the substantially parallel and vertical electric fields generate together at corners of the electrode.
In the IPS mode LCD device, it is possible to control the alignment of liquid crystal molecules by using the electric field. For example, as shown in FIG. 3A, if a voltage is applied to liquid crystal molecules 31 initially aligned in the same direction as a transmission axis of one polarizing sheet, longitudinal axes of the liquid crystal molecules 31 are aligned substantially parallel to the electric field, as shown in FIG. 3B. Specifically, first and second polarizing sheets are formed on outer surfaces of the lower and upper substrates, wherein transmission axes of the first and second polarizing sheets are arranged perpendicular to each other. As an orientation layer of the lower substrate is rubbed substantially parallel to the transmission axis of any one polarizing sheet, it is displayed as a normally black mode. That is, if the voltage is not applied to the device, the liquid crystal molecules 31 are aligned as shown in FIG. 3A, thereby displaying the black state. If the voltage is applied to the device, as shown in FIG. 3B, the liquid crystal molecules 31 are aligned substantially parallel to the electric field, thereby displaying the white state. In FIG. 3A and FIG. 3B, non-explained reference numbers ‘24’ and ‘17’ respectively represent the common electrode and the pixel electrode. Accordingly, the IPS mode LCD device has a wide viewing angle as compared with that of the TN mode LCD device.
A method for fabricating the aforementioned LCD device will be described in detail. The TN mode LCD device, the transflective mode LCD device and the IPS mode LCD device have similar fabrication processes. A method for fabricating the IPS mode LCD device will be described as follows. FIG. 4A to FIG. 4D are cross-sectional views illustrating the fabrication process of the IPS mode LCD device according to the related art.
As shown in FIG. 4A, a low-resistance metal layer is deposited on a lower substrate 11 by sputtering, and then patterned, thereby forming a gate line (not shown) and a gate electrode 12a. A common line (not shown) parallel to the gate line, and a plurality of common electrodes 24 diverged from the common line are formed at the same time. After that, a gate insulating layer 13 is formed in a method of depositing a silicon nitride layer SiNx on an entire surface of the lower substrate 11 including the gate line. Then, an amorphous silicon layer is deposited on the entire surface of the lower substrate 11, and then selectively removed, thereby forming a semiconductor layer 14 on the gate insulating layer 13 above the gate electrode 12a. 
Referring to FIG. 4B, a low-resistance metal layer is deposited on the gate insulating layer 13 by sputtering, and then patterned to form a data line (not shown) and source/drain electrodes 15a and 15b. Subsequently, a plurality of pixel electrodes 17 are connected with the drain electrode 15b, and formed in parallel to the data line. The pixel electrodes 17 are arranged between each common electrode 24, whereby the pixel electrode 17 alternates with the common electrode 24. At this time, the pixel electrode 17 may be formed at the same time as the data line of metal, or may be formed additionally by using a transparent conductive layer such as ITO. Also, the pixel electrode 17 and the common electrode 24 may be formed in a straight pattern or a zigzag pattern. Thereafter, as shown in FIG. 4C, a passivation layer 16 is formed in a method of depositing or coating a silicon nitride layer or an organic insulating layer of BCB on the entire surface of the lower substrate 11 including the data line 15. Also, a first orientation layer 50 is formed on the passivation layer 16, and then rubbed.
As shown in FIG. 4D, a black matrix layer 22 of metal such as Cr or CrOx is formed on an upper substrate 21 to prevent light leakage, and R/G/B color filter layers 23 are formed between each black matrix layer 22 in an electrodeposition method, a pigment spray method or a coating method, to realize various colors. Then, a second orientation layer 60 is deposited thereon. Also, a sealant (not shown) is formed on the lower substrate 11 or the upper substrate 21, and spacers (not shown) are formed on any one of the two substrates 11 and 21. In this state, the two substrates 11 and 21 facing each other are bonded to each other. Then, liquid crystal 30 is injected between the lower and upper substrates 11 and 21 bonded to each other, and first and second polarizing sheets 81 and 82 are respectively formed on outer surfaces of the lower and upper substrates 11 and 21, thereby completing the IPS mode LCD device according to the related art. At this time, the transmission axes of the first and second polarizing sheets 81 and 82 are substantially perpendicular to each other, and one transmission axis is at the same direction as the electric field.
The rubbing process will be described in detail. FIG. 5 is a cross-sectional view explaining the rubbing process according to the related art. The rubbing process includes the sequential process of forming the organic high polymer layer referred to as the orientation layer on the substrate, and obtaining anisotropy therein. That is, polyamic acid or soluble polyimide is coated on the substrate, and sequentially cured at a temperature between 60° C. and 80° C. and between 80° C. and 200° C., whereby the coated polyamic acid or soluble polyimide is formed to a polyimide layer. As shown in FIG. 5, the polyimide layer is rubbed with a cylindrical rubbing roll 70. At this time, the rubbing process is progressed by rotating the cylindrical rubbing roll 70 coated with a rubbing cloth 71 such as nylon or rayon, so that the surface of the polyimide layer is rubbed mechanically. However, a perpendicular band or a horizontal band is generated by seams of the rubbing cloth 71 of the rubbing roll 70. Also, end portions of the rubbing cloth may come off the rubbing roll 70.
The aforementioned IPS mode LCD device according to the related art has the following disadvantages.
FIG. 6 is a photograph illustrating light leakage in a surface of step coverage in an LCD device according to the related art. FIG. 7 is a photograph illustrating light leakage in a surface having no step coverage in an LCD device according to the related art. Referring to FIG. 6, the thin film transistor array substrate in each mode has the step coverage on the surface thereof. That is, in case of the TN mode LCD device, the thin film transistor portion and the crossing portion of the gate and data lines are relatively higher than the other portions of the thin film transistor array substrate. In a case of the transflective type LCD device, the step coverage is generated between the transmitting part and the reflective part in the pixel region. Also, in the IPS mode LCD device, the step coverage of approx. 2500 Å is generated by the pattern of common electrode 24 and pixel electrode 17. During the rubbing process of the orientation layer 50, the rubbing cloth 71 is not in contact with the relatively low portion of the step coverage, thereby generating the defective rubbing. Also, in the IPS mode LCD device using three masks, step coverage of approx. 8000 Å is generated at the contact portion of the pixel electrode and the drain electrode of the thin film transistor. During the rubbing process of the orientation layer, the rubbing cloth is not in contact with the relatively low portion of the step coverage, thereby generating the defective rubbing. Referring to FIG. 6, the light leakage is generated in the initial black state because it is impossible to control the alignment of liquid crystal at the portion having no alignment pattern. The picture quality is deteriorated due to low contrast ratio. Meanwhile, the light leakage is generated in the surface having no step coverage due to the defective rubbing because the alignment pattern is not uniform due to non-uniformity of the rubbing cloth.